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<a name="Elimination"></a>
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<hr>
<a name="Eliminating-Frame-Pointer-and-Arg-Pointer"></a>
<h4 class="subsection">18.9.5 Eliminating Frame Pointer and Arg Pointer</h4>

<p>This is about eliminating the frame pointer and arg pointer.
</p>
<dl>
<dt><a name="index-TARGET_005fFRAME_005fPOINTER_005fREQUIRED"></a>Target Hook: <em>bool</em> <strong>TARGET_FRAME_POINTER_REQUIRED</strong> <em>(void)</em></dt>
<dd><p>This target hook should return <code>true</code> if a function must have and use
a frame pointer.  This target hook is called in the reload pass.  If its return
value is <code>true</code> the function will have a frame pointer.
</p>
<p>This target hook can in principle examine the current function and decide
according to the facts, but on most machines the constant <code>false</code> or the
constant <code>true</code> suffices.  Use <code>false</code> when the machine allows code
to be generated with no frame pointer, and doing so saves some time or space.
Use <code>true</code> when there is no possible advantage to avoiding a frame
pointer.
</p>
<p>In certain cases, the compiler does not know how to produce valid code
without a frame pointer.  The compiler recognizes those cases and
automatically gives the function a frame pointer regardless of what
<code>targetm.frame_pointer_required</code> returns.  You don&rsquo;t need to worry about
them.
</p>
<p>In a function that does not require a frame pointer, the frame pointer
register can be allocated for ordinary usage, unless you mark it as a
fixed register.  See <code>FIXED_REGISTERS</code> for more information.
</p>
<p>Default return value is <code>false</code>.
</p></dd></dl>

<dl>
<dt><a name="index-ELIMINABLE_005fREGS"></a>Macro: <strong>ELIMINABLE_REGS</strong></dt>
<dd><p>This macro specifies a table of register pairs used to eliminate
unneeded registers that point into the stack frame.
</p>
<p>The definition of this macro is a list of structure initializations, each
of which specifies an original and replacement register.
</p>
<p>On some machines, the position of the argument pointer is not known until
the compilation is completed.  In such a case, a separate hard register
must be used for the argument pointer.  This register can be eliminated by
replacing it with either the frame pointer or the argument pointer,
depending on whether or not the frame pointer has been eliminated.
</p>
<p>In this case, you might specify:
</p><div class="smallexample">
<pre class="smallexample">#define ELIMINABLE_REGS  \
{{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
 {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
 {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}
</pre></div>

<p>Note that the elimination of the argument pointer with the stack pointer is
specified first since that is the preferred elimination.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fCAN_005fELIMINATE"></a>Target Hook: <em>bool</em> <strong>TARGET_CAN_ELIMINATE</strong> <em>(const int <var>from_reg</var>, const int <var>to_reg</var>)</em></dt>
<dd><p>This target hook should return <code>true</code> if the compiler is allowed to
try to replace register number <var>from_reg</var> with register number
<var>to_reg</var>.  This target hook will usually be <code>true</code>, since most of the
cases preventing register elimination are things that the compiler already
knows about.
</p>
<p>Default return value is <code>true</code>.
</p></dd></dl>

<dl>
<dt><a name="index-INITIAL_005fELIMINATION_005fOFFSET"></a>Macro: <strong>INITIAL_ELIMINATION_OFFSET</strong> <em>(<var>from-reg</var>, <var>to-reg</var>, <var>offset-var</var>)</em></dt>
<dd><p>This macro returns the initial difference between the specified pair
of registers.  The value would be computed from information
such as the result of <code>get_frame_size ()</code> and the tables of
registers <code>df_regs_ever_live_p</code> and <code>call_used_regs</code>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fCOMPUTE_005fFRAME_005fLAYOUT"></a>Target Hook: <em>void</em> <strong>TARGET_COMPUTE_FRAME_LAYOUT</strong> <em>(void)</em></dt>
<dd><p>This target hook is called once each time the frame layout needs to be
recalculated.  The calculations can be cached by the target and can then
be used by <code>INITIAL_ELIMINATION_OFFSET</code> instead of re-computing the
layout on every invocation of that hook.  This is particularly useful
for targets that have an expensive frame layout function.  Implementing
this callback is optional.
</p></dd></dl>

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